The field of photovoltaics generally relates to multi-layer materials that convert sunlight directly into DC electrical power. The basic mechanism for this conversion is the photovoltaic (or photoelectric) effect, first correctly described by Einstein in a seminal 1905 scientific paper for which he was awarded a Nobel Prize for physics. In the United States, photovoltaic (PV) devices are popularly known as solar cells. Solar cells are typically configured as a cooperating sandwich of p-type and n-type semiconductors, in which the n-type semiconductor material (on one “side” of the sandwich) exhibits an excess of electrons, and the p-type semiconductor material (on the other “side” of the sandwich) exhibits an excess of holes, each of which signifies the absence of an electron. Near the p-n junction between the two materials, valence electrons from the n-type layer move into neighboring holes in the p-type layer, creating a small electrical imbalance inside the solar cell. This results in an electric field in the vicinity of the junction.
When an incident photon excites an electron in the cell into the conduction band, the excited electron becomes unbound from the atoms of the semiconductor, creating a free electron/hole pair. Because, as described above, the p-n junction creates an electric field in the vicinity of the junction, electron/hole pairs created in this manner near the junction tend to separate and move away from junction, with the electron moving toward the n-type side, and the hole moving toward the p-type side of the junction. This creates an overall charge imbalance in the cell, so that if an external conductive path is provided between the two sides of the cell, electrons will move from the n-type side back to the p-type side along the external path, creating an electric current. In practice, electrons may be collected from at or near the surface of the n-type side by a conducting grid that covers a portion of the surface, while still allowing sufficient access into the cell by incident photons.
Such a photovoltaic structure, when appropriately located electrical contacts are included and the cell (or a series of cells) is incorporated into a closed electrical circuit, forms a working PV device. As a standalone device, a single conventional solar cell is not sufficient to power most applications. As a result, solar cells are commonly arranged into PV modules, or “strings,” by connecting the front of one cell to the back of another, thereby adding the voltages of the individual cells together in electrical series. Typically, a significant number of cells are connected in series to achieve a usable voltage. The resulting DC current then may be fed through an inverter, where it is transformed into AC current at an appropriate frequency, which is chosen to match the frequency of AC current supplied by a conventional power grid. In the United States, this frequency is 60 Hertz (Hz), and most other countries provide AC power at either 50 Hz or 60 Hz.
One particular type of solar cell that has been developed for commercial use is a “thin film” PV cell. In comparison to other types of PV cells, such as crystalline silicon PV cells, thin film PV cells require less light-absorbing material to create a working cell, and thus can reduce processing costs. Thin film based PV cells also offer improved cost by employing previously developed deposition techniques widely used in the thin film industries for protective, decorative, and functional coatings. Common examples of low cost commercial thin film products include water permeable coatings on polymer-based food packaging, decorative coatings on architectural glass, low emissivity thermal control coatings on residential and commercial glass, and scratch and anti-reflective coatings on eyewear. Adopting or modifying techniques that have been developed in these other fields has allowed a reduction in development costs for PV cell thin film deposition techniques.
Furthermore, thin film cells, particularly those employing a sunlight absorber layer of copper indium diselenide, copper indium disulfide, copper indium aluminum diselenide, or copper indium gallium diselenide, have exhibited efficiencies approaching 20%, which rivals or exceeds the efficiencies of the most efficient crystalline cells. In particular, copper indium gallium diselenide (CIGS) is stable, has low toxicity, and is truly thin film, requiring a thickness of less than two microns in a working PV cell. As a result, to date CIGS appears to have demonstrated the greatest potential for high performance, low cost thin film PV products, and thus for penetrating bulk power generation markets.
Thin film PV materials may be deposited either on rigid glass substrates, or on flexible substrates. Glass substrates are relatively inexpensive, generally have a coefficient of thermal expansion that is a relatively close match with the CIGS or other absorber layers, and allow for the use of vacuum deposition systems. However, such rigid substrates suffer from various shortcomings, such as a need for substantial floor space for processing equipment and material storage, specialized heavy duty handling equipment, a high potential for substrate fracture, increased shipping costs due to the weight and delicacy of the glass, and difficulties in installation. As a result, the use of glass substrates is not optimal for large-volume, high-yield, commercial manufacturing of multi-layer functional thin film materials such as photovoltaics.
In contrast, roll-to-roll processing of thin flexible substrates allows for the use of compact, less expensive vacuum systems, and of non-specialized equipment that already has been developed for other thin film industries. PV cells based on thin flexible substrate materials also exhibit a relatively high tolerance to rapid heating and cooling and to large thermal gradients (resulting in a low likelihood of fracture or failure during processing), require comparatively low shipping costs, and exhibit a greater ease of installation than cells based on rigid substrates.
A particular type of n-type semiconductor material that may be used in thin-film PV cells is known in the field of chemistry as a chalcogenide. A chalcogenide is a chemical compound consisting of at least one chalcogen ion and at least one more electropositive element such as a metal. Forming a thin film of chalcogenide is described in the prior art, for example, in U.S. Pat. No. 6,537,845 to McCandless et al., which is hereby incorporated into the present disclosure by reference for all purposes. However, forming chalcogenide films having a desired thickness and uniformity remains technically challenging and improvements are needed.
Historically, the formation of a thin-film chalcogenide buffer layer or layers often proceeds by a relatively inefficient cyclical process that includes heating the substrates in a water-containing vessel to an elevated temperature, adding and mixing in a metallic salt, and then adding and mixing in a chalcogen-containing component. After a proscribed time at a proscribed temperature, the reaction is complete, the substrates are removed, the used solution is sent to waste treatment, reactant-containing solution is applied to the web, and the vessel is cleaned for the next reaction. In addition, existing methods of applying the reactant-containing solution to the web typically result in deposition of chalcogenide over both the desired (“front” or “top”) surface of the web, and also over at least a portion of the other (“back” or “bottom”) surface of the web, requiring at least one cleaning step to remove the material from the back surface. This is typically accomplished with an acidic solution that must be carefully controlled and completely removed to avoid damage to the desired thin-film layers and to avoid long-term corrosion issues activated by the presence of residual acidity.
Furthermore, when reactant solutions reach the back side of the substrate, either because the entire substrate is immersed in a bath of reactant solution, or because solutions applied to the top side of the substrate are insufficiently contained on the top side, it may be difficult or impossible to remove all of the excess chalcogenide that forms on the back of the substrate and/or any heating elements. This can affect the amount of heat reaching the top of the substrate. For example, chalcogenide build-up on the underside of the substrate may affect the heat capacity of the substrate and its thermal conductivity, and chalcogenide build-up on the heaters may affect the emissivity and/or thermal conductivity of the heaters. Furthermore, these effects may be non-uniform across the width of the substrate.
Such undesirable chalcogenide deposition on the underside of the substrate and/or on any heating elements disposed on that side of the substrate may result in a poorly controlled substrate temperature and the corresponding formation of a chalcogenide buffer layer having undesirable characteristics. For example, if the amount of heat reaching the top surface of the substrate is decreased due to chalcogenide formation under the substrate, this may result in the formation of an undesirably thin chalcogenide buffer layer on the top surface. Similarly, if the amount of heat reaching the top surface of the substrate is non-uniform due to non-uniform chalcogenide formation under the substrate, this may result in the formation of an undesirably non-uniform chalcogenide buffer layer on the top surface. These effects may be difficult to control due to unpredictable and uncontrolled chalcogenide formation under the substrate.
It is known in the art to deposit a chalcogenide layer on a substrate web in a roll-to-roll process and to raise the lateral edges of the substrate web, for example by draping the web edges over vertical rails, to improve solution containment. However, this typically leads to undesirable buckling forces on the web, making it difficult to maintain a desired degree of flatness in the deposition region. At the same time, previous systems used a hold-down mechanism such as one or more wheels to keep the web in contact with an underlying surface such as a conductive heater. However, this creates local hot spots on the substrate, resulting in undesirable nonuniformities in the chalcogenide layer. It is also known in the art to tilt the entire web longitudinally to control the depth of reactant solutions on the web. However, these prior systems do not provide for multiple slope adjustments in various portions of the deposition region, and thus may not provide sufficient control over the solution depth and the corresponding chalcogenide thickness. For all of the above reasons, improved methods and apparatus for containing chalcogenide reactants to the front or top surface of a substrate are desirable.